hydraulic motor device for assisting the mechanical transmission of a vehicle

ABSTRACT

A hydraulic motor device including a hydraulic motor having a cylinder block and a cam that are mounted to move in rotation relative to each other. The motor has a fluid distributor for the cylinders, a portion of which distributor is constrained in rotation with the cam, and a stationary distribution coupling connecting the distributor to a fluid feed and to a fluid discharge. The motor has a through passage disposed along the axis of rotation for the purpose of receiving a transmission shaft segment. The motor can be declutched by radially uncoupling the pistons from the cam, the cylinder block remaining stationary relative to the reference frame of the vehicle, and the cam being constrained in rotation with a transmission shaft segment that is disposed in the through passage. The device may be used in addition to or as a replacement for a mechanical transmission.

The present invention relates to a hydraulic motor device, in particular for assisting the mechanical transmission of a vehicle, which hydraulic motor device includes a hydraulic motor that comprises a cylinder block having cylinders slidably receiving radial pistons, and a reaction cam for the pistons, the cylinder block and the cam being mounted to move in rotation relative to each other about an axis of rotation, the motor further comprising a fluid distributor for the cylinders, a portion of which distributor is constrained in rotation with the cam, and a distribution coupling mounted to be stationary relative to a reference frame of the vehicle and for connecting the distributor to a fluid feed and to a fluid discharge, the motor having a through passage disposed along the axis of rotation for the purpose of receiving a transmission shaft segment.

Such devices can be used to drive certain vehicles, such as, for example, combine harvesters or other farm or worksite vehicles.

Such a vehicle, in addition to requiring a high speed serving for driving the vehicle on the road (“road mode”), also need a high-torque low speed for driving the vehicle at low speed with a view to doing a specific piece of work (“work mode”). In work mode, the drive applied to the vehicle can require very fine control of speed or of path, moving the vehicle at low speed, absence of wheel spin, and, above all, very high torque.

In order to make it possible to use these two modes, such vehicles are generally equipped with a hybrid, mechanical and hydraulic transmission. Such a transmission is made up of a mechanical main transmission suitable for transmitting the drive torque from a main engine or motor via mechanical links, and of a hydraulic auxiliary transmission including a hydraulic motor device such as the device presented in the introduction.

In the hydraulic auxiliary transmission, a fraction of the power delivered by the main engine or motor (generally an internal combustion engine) is converted by means of the hydraulic motor device into a fluid pressure and is transferred to a hydraulic motor. The hydraulic motor in turn relays drive torque to the main mechanical transmission, in particular for low rotation speeds, replacing the torque delivered by the main engine.

Advantageously, such a drive system makes it possible for the vehicle to have the desired two different modes of use.

In road mode, drive is applied to the vehicle in conventional manner via the mechanical transmission that transmits the torque from the main engine to the wheels. The hydraulic motor device is then inactive and the hydraulic motor is declutched from the mechanical transmission.

Conversely, in work mode, drive via the main engine only is inappropriate because it would require the speed of rotation of the outlet shaft of said engine to be geared down excessively, and would not generate the required torque.

In such a situation, the hydraulic motor device is used. Its radial-piston hydraulic motor offers the required capabilities for applying drive to the vehicle in work mode, and for doing the work in question under good conditions, because it is particularly suitable for delivering high torque at a low speed of rotation. The rotary drive torque is then delivered by the hydraulic motor, replacing the torque transmitted in road mode to the mechanical main transmission by the main engine.

Patent FR 2 739 418 presents such an assistance hydraulic motor. In the device presented by that patent, the motor has a stationary casing that has its inside face arranged to form a cam, and houses a cylinder block mounted to rotate relative to the casing. The cylinder block is secured to a positive clutch so as to enable the hydraulic motor to be clutched or declutched so as to drive, or not drive, a mechanical transmission shaft that passes through the motor.

In such a device, it is necessary for the mechanical transmission shaft to be at a standstill or to be rotating very slowly for the clutching or declutching by the positive clutch to take place, and that is not very practical. In addition, it complicates the structure of the motor and increases its dimensions, by making it necessary for the positive clutch element that is driven by the cylinder block to be mounted to move axially along the axis of the mechanical transmission shaft in order to engage with or disengage from the teeth of the positive clutch.

In addition, when the hydraulic motor is in engagement (via the positive clutch) with the mechanical transmission, the maximum speed of rotation is limited, and, with the motor as described, it is not possible to uncouple the hydraulic motor by retracting the pistons. For that reason, in practice, as drive means for driving the rotary portions of the mechanical transmission directly in rotation, it is necessary to use either the internal combustion engine, or the hydraulic motor, but not both at the same time.

Another solution consists in using a hydraulic motor device that has a rotary cylinder block engaged with a transmission shaft segment, and whose pistons can be held in positions in which they are uncoupled from the cam by being retracted into their cylinders, so as to maintain the motor in the declutched configuration. However, since the cylinder block is in engagement with the transmission shaft segment, it rotates with said shaft even when the pistons are uncoupled, thereby subjecting said pistons to centrifugal forces that tend to make them come out of the cylinders. For the declutched configuration, it is thus necessary to balance said forces by fluid pressures that hold the pistons in the positions in which they are retracted in the cylinders. As a result, the means necessary for operation in the declutched configuration are relatively complex.

Thus, with that solution also, as drive means for driving the rotary portions of the mechanical transmission directly in rotation, it is necessary to use either the internal combustion engine, or the hydraulic motor, but not both at the same time.

A first object of the present invention is to design a device of the type presented in the introduction, and in which the hydraulic motor can be declutched from the mechanical transmission, or can deliver additional torque thereto, or can operate as a replacement therefore, while also remaining relatively simple, and in particular without having a positive clutch.

This object is achieved by the fact that the motor can take up a declutched configuration in which the pistons are radially uncoupled from the cam, the cylinder block is stationary relative to the reference frame of the vehicle, while the cam is free to rotate, and the cam is constrained in rotation with a transmission shaft segment that is disposed in said through passage.

In the cylinder block, when the pistons are in the “retracted” position, i.e. when the cam is in the uncoupled position, the motor is in declutched mode. In this mode, since the cylinder block is stationary, the pistons are not subjected to any centrifugal force, and this results in the pistons being in a stable uncoupled position. This result cannot be obtained with the above-mentioned motor having a rotary cylinder block disposed inside a stationary cam.

The fact that the transmission shaft segment and the cam are constrained in rotation is generally used to transmit drive torque from the cam to the transmission shaft segment. However, the cam or optionally an intermediate part connected to the cam may itself be part of the mechanical transmission, and thus receive the drive torque transmitted by the transmission shaft segment, so as to transmit it to drive members for moving the vehicle, for example. In which case, the drive torque generated by the hydraulic motor may be transmitted directly by the cam to said drive members, without going via the transmission shaft segment.

Finally, in the hydraulic motor, the cylinder block is stationary relative to the reference frame of the vehicle, i.e. the cylinder block does not rotate. As a result, no centrifugal force is applied to the pistons of the cylinder block. It is thus easy to keep the motor in the declutched configuration.

In addition, by means of the invention, it is possible to clutch or declutch the hydraulic motor, even if the transmission shaft segment is rotating at a significant speed.

It should be noted, in particular, that, compared with a purely mechanical transmission that is designed mainly for road mode, the hydraulic motor device of the invention considerably simplifies the gearbox and the transmission, by avoiding any need to provide additional reducing gear stages for the working speeds that are very slow.

Conversely, mechanical transmissions that are designed mainly for the work mode do not offer good efficiency in road mode. The invention makes it possible to avoid this problem, by providing a very good transmission for the work mode, and a transmission that is optimized for road mode. More precisely, with the invention, the hydrodynamic drag of the hydraulic motor is very low, and thus does not adversely affect the efficiency of the transmission when said transmission is in road mode.

In an embodiment, the device further includes a drive part secured to the cam and that is suitable for co-operating with said transmission shaft segment to drive said shaft segment in rotation.

For example, the drive part may be provided with internal fluting on one end, which fluting is suitable for co-operating with complementary fluting on the transmission shaft segment to secure the part to the transmission shaft segment.

Since the drive part transfers the drive torque from the cam to the transmission shaft segment, the cam thus remains of small dimensions, thereby enabling it to be replaced at lower cost, and reducing its manufacturing cost, since the manufacturing constraints for machining the cam are particularly stringent.

In an embodiment, the drive part and the transmission shaft segment are formed integrally as a single part. Thus, the transmission is simplified by means of it having a small number of parts, and the risk of malfunctioning at the junction between the drive part and the transmission shaft segment is eliminated.

In an embodiment, on its periphery, the hydraulic motor has a connection portion adapted to transmit torque, e.g. a flange. Thus, instead of the torque being transmitted substantially in the vicinity of the axis of the transmission shaft segment, the torque is transmitted via a connection portion that is of larger diameter. Thus the mechanical stresses received by said connection portion are small. The connection portion can be situated on any part of the motor that is constrained to move with the cam, such as the drive part, the cam itself, or any other portion of the casing of the motor that is constrained to move with the cam. Transmitting the torque via such a connection portion rather than via one end of the transmission shaft segment also advantageously makes it possible to increase the compactness of the transmission by reducing its overall axial size.

In an embodiment, the distribution coupling has a sleeve portion whose inside periphery forms a portion of the through passage. Depending on the embodiment, this portion may be elongate to a greater or to a lesser extent. It can also extend over a distance sufficient to pass through the distributor and through the cylinder block. The distribution coupling can then serve in particular, at least in part by means of its bearings, to fasten and/or to position the motor on the shaft segment. In which case, the dual function of the coupling, namely the function of conveying the feed and discharge fluids for the motor together with the function of holding the motor, contributes to the compactness of the hydraulic motor device.

A second object of the present invention is to design a vehicle drive system including a mechanical transmission operated by at least one main engine or motor, and that has a road mode in which the drive torque is delivered mainly by the main engine, and a work mode in which the drive torque is delivered by one or more hydraulic motor devices, in addition to or as a replacement for the main engine, the drive system remaining relatively simple.

This object is achieved by the fact that the vehicle drive system includes a device as defined above, with its transmission shaft being a portion of the mechanical transmission.

The invention can be well understood and its advantages appear more clearly on reading the following detailed description of embodiments shown by way of non-limiting example. The description refers to the accompanying drawings, in which:

FIGS. 1A and 2 are axial section views of assistance hydraulic motor devices of the invention, in first and second embodiments of the invention;

FIG. 1B is a fragmentary axial section view of a variant of the hydraulic motor device of FIG. 1A;

FIG. 3 is a radial section view of the motor through the cylinder block in the second embodiment, on the plane of FIG. 2; and

FIGS. 4 and 5 are diagrammatic views from above of vehicle drive systems of the invention, in two different embodiments.

A first embodiment of a hydraulic motor device of the invention is described below with reference to FIG. 1A.

The hydraulic motor device 500 of FIG. 1A comprises:

a mechanical transmission shaft 200 carrying coupling plates at its ends, namely an inlet coupling plate 202 at one end, and an outlet plate 204 at the other end, with a view to connecting it to the other segments of the mechanical transmission of the vehicle; and

a hydraulic motor 100 having a through passage 102 through which the shaft 200 passes.

The hydraulic motor 100 comprises:

a cylinder block 10 in which radial pistons 12 are slidably received;

a casing assembly in three portions 20, 30, 40 that are fastened together by screws 22, in which casing assembly the first portion 20 is a reaction cam mounted to rotate about the axis of the shaft 200, and caused to move in rotation by the action of the pistons 12 on its undulating inside cam surface 24; the second portion 30 is a drive part that transfers the torque received by the cam 20 to the shaft 200 via fluting 32; the third portion 40 is a distribution casing;

a distribution coupling 50 mounted in stationary manner on the chassis of the vehicle (not shown) via a flange portion 52, and also having a sleeve portion 54 extending around the transmission shaft 200;

a distributor 60 mounted to move in rotation about a circularly symmetrical surface 56 of said sleeve portion 54, synchronized in rotation with the distribution casing 40 by means of spring screws 42, and therefore constrained in rotation with the cam 20.

In the cylinder block, the cylinders extend radially relative to the axis of rotation A and the cam surface 24 of the reaction cam 20 is disposed around the cylinder block 10, and faces towards the axis of rotation (A).

The distribution coupling 50 is connected via fluid feed and fluid discharge orifices 58 to fluid feed and fluid discharge ducts (not shown) of the vehicle.

The distributor 60 has distribution ducts 62 that are connected respectively to the feed and to the discharge via the ducts 55 in the distribution coupling 50, and via two grooves 51 arranged between the distribution coupling 50 and the distributor 60. The cylinder ducts 13 communicate with the distribution ducts 62 as the distributor moves in rotation so as to connect the cylinders alternately to the feed and to the discharge.

The hydraulic motor 100 is held on the transmission shaft 200 by rotary bearings 103 and 104 that are disposed in the sleeve portion 54 of the distribution coupling 50 around the shaft 200. To this end, the sleeve portion co-operates with at least one rotary support bearing for supporting the shaft segment in rotation relative to the motor. It thus holds the shaft segment 200 in position where it passes through the hydraulic motor. The ball bearings 103 and 104 are spaced apart from each other and are placed substantially on either side of the hydraulic motor, so as to limit the radial forces to which they can be subjected. Thus, in the embodiment shown, one of the bearings is situated in the vicinity of the fluid feed and of the fluid discharge (orifices 58), and the other bearing is situated at the same level (along the axis) as the cylinder block 10, inside the sleeve portion 54 that extends in an internal passage 14 of the cylinder block 10.

Fluid is prevented from leaking between the distribution coupling 50 and the transmission shaft 200 by an annular lip seal 57 disposed around the shaft 200, at the same end as the inlet coupling plate 202. In this first embodiment (FIG. 1), the distributor 60 is disposed about the sleeve portion 54. It is situated on the same side of the cylinder block as the fluid feed and discharge (orifices 58).

The distribution casing 40 is fastened to the cam 20 by the screws 22, and is guided in rotation about the sleeve portion 54 by a rotary support bearing 44.

Holding pieces 207 are screwed onto the ends of the shaft 200 for the purpose of holding the coupling plates 202, 204 axially by means of washers 208.

In the embodiment of FIG. 1A, the drive torque delivered at the outlet under the combined actions of the main engine and/or of the hydraulic motor is transmitted by the outlet coupling plate 204 (on the left of FIG. 1A), the transmission shaft segment 200 receiving the torque coming from the main engine via the inlet coupling plate 202.

In another embodiment, the drive torque can be delivered at the outlet via the motor itself, at a connection portion situated on the periphery of the motor (relative to its axis of rotation) and adapted to transmit torque. This connection portion may be part of any piece of the casing assembly, e.g. of the drive part 30, of the reaction cam 20, or indeed of the distribution casing 40.

Operation of the hydraulic motor device is as follows:

When the hydraulic motor is in drive mode, said motor receives and removes the hydraulic fluid under pressure via the feed and discharge orifices 58. The fluid goes via the internal ducts 55 in the distribution coupling 50, is exchanged with the distributor 60 via the grooves 51, passes through the distributor 60 via the ducts 62, and is finally exchanged with the cylinders via the cylinder ducts 13.

Due to the distributor 60 rotating relative to the cylinder block 10, the cylinders are periodically partially filled and emptied, and the fluid pressure varies periodically in the cylinders. As a result, the pistons 12 of the motor are pressed against the cam surface 24 in such a manner as to develop drive torque, transmitted to the casing assembly. Since the casing assembly is constrained to rotate with the shaft 200, said shaft is driven in rotation by the hydraulic motor.

In reaction to the drive torque, the cylinder block 12 receives opposing torque of direction opposite to the direction of the drive torque. The cylinder block is provided with fluting 16 in its internal passage 14, which fluting meshes with corresponding fluting 53 of the distribution coupling. The opposing torque is thus transmitted to the distribution coupling that in turn transmits those forces to the remainder of the vehicle via its flange portion 52.

In order to cause the hydraulic motor to go from the drive mode to the declutched mode, it suffices merely to interrupt feeding the hydraulic motor with pressurized fluid. Under such conditions, there is no longer any force urging the pistons radially to engage them against the cam. The pistons therefore remain in the position in which they are uncoupled from the cam, and the hydraulic motor is thus at rest, the casing being free to rotate in freewheeling manner about the cylinder block and about the distribution coupling.

Conversely, in order to cause the hydraulic motor to go from the declutched mode to the drive mode, it is necessary to go via an acceleration phase during which the movement in rotation of the casing assembly is accelerated, and which makes it possible to synchronize the speeds and the relative positions of the casing assembly of the shaft 200.

A variant of the hydraulic motor device shown in FIG. 1A is described below with reference to FIG. 1B.

In the hydraulic motor device of FIG. 1B, the transmission shaft segment and the drive part (that are referenced 200 and 30 in FIG. 1A) are in the form of a single part 20030. Thus, the fluting for connecting together the two parts is eliminated, as are the manufacturing constraints and any problems of slack that might arise in such a connection by fluting.

A second embodiment of a hydraulic motor device of the invention is presented below with reference to FIGS. 2 and 3.

The hydraulic motor device 500B of FIGS. 2 and 3 has a mechanical transmission shaft 200B and a hydraulic motor 100B.

The hydraulic motor 100B has a cylinder block 10B, a casing assembly in three portions 20B, 30B, 40B, a distribution coupling 50B, and a distributor 60B.

Unless otherwise indicated, the various components of the hydraulic motor 100B have relative positions and functions similar to those of the corresponding portions of the hydraulic motor 100 of the first embodiment of the invention.

The main structural difference is due to the hydraulic fluid distribution system being modified. The distribution coupling 150B is no longer a single part 50 as it is in the motor 100, but rather it is made up of three main parts 50B, 70B, 80B.

Thus, instead of having fluid exchange between the fluid admission and fluid return orifices 58 and the distributor via a single part 50 (the distribution coupling 50 of the motor 100), in the motor 100B this fluid exchange takes place via the three parts 50B, 70B, 80B of the distribution coupling 150B.

The parts 70B and 80B are disposed on either side of the cylinder block 10B. They are provided with respective internal channels 75B and 85B, via which the hydraulic fluid flows.

The cylinder block 10B and the parts 70B and 80B are fastened together by screws 152B that are disposed axially in passages 17B, 77B, 87B provided in these parts.

Like the part 80B, the distributor is disposed on the side of the cylinder block opposite from its side facing the fluid feed and to the fluid discharge.

The advantage of this configuration is that the port plate 12 is situated opposite from the fluid feed and the fluid discharge of the motor. Thus, the fluid pressure exerted on the port plate tends advantageously to push the cylinder block towards the fluid feed and the fluid discharge of the motor, i.e. towards the chassis of the vehicle. The motor 100B thus tends to remain compact and to limit its leaks.

The section view of FIG. 3 shows a section through the hydraulic motor 100B at the cylinder block 10B. The cylinder block has ten cylinders 18B. The following are arranged between the cylinders:

eight passages 17B for the screws 152B; and

two passages 15B respectively for hydraulic fluid feed and for hydraulic fluid discharge.

Thus, the distributor is connected to the fluid feed and to the fluid discharge via ducts passing through the cylinder block between the cylinders. Since the cylinder block is stationary, the empty space between the cylinders is advantageously used to cause hydraulic fluid to pass through the ducts, thereby avoiding increasing the diameter of the cylinder block.

Two drive systems (1001, 1002) for a vehicle (not shown), each of which systems includes a hydraulic motor device 500 of the invention, are presented below with reference to FIGS. 4 and 5.

These systems drive wheels 630 by transmitting the power delivered by an internal combustion main engine 400. Each of these systems comprises a mechanical main transmission 800 and a hydraulic auxiliary transmission. Naturally, the vehicle can have other wheels and other additional transmission members.

The mechanical main transmission 800 comprises a mechanical gearbox 810, a main transmission shaft 820, and an axle 830.

Said mechanical main transmission 800 transmits the drive torque output from the motor 400 to the wheels 630 in the following manner:

The motor 400 is coupled to the gearbox 810, to which it communicates drive torque. The gearbox 810 delivers said drive torque on its outlet shaft 812. This shaft 812 is connected to a first end 825 of a main transmission shaft 820 that it drives in rotation. The main transmission shaft 820 is connected at its other end 826 to an axle 830. The axle 830 comprises a power divider 832 and two half-shafts 834.

The power divider 832 that includes a differential, receives the drive torque from the main transmission shaft 820 and transmits it to the two half-shafts 834. Said half-shafts are coupled to the wheels 630 to which they communicate the drive torque.

In addition to the mechanical main transmission, the drive systems 1001 and 1002 also include a hydraulic auxiliary transmission.

This hydraulic auxiliary transmission comprises:

at least one assistance hydraulic motor device 500;

a hydraulic pump;

a secondary transmission shaft for transmitting mechanical power to the hydraulic pump for the purpose of driving said pump; and

a hydraulic circuit connecting the hydraulic pump to the feed and to the discharge of the hydraulic motor device(s) 500 for the purpose of feeding said device(s) with hydraulic fluid under pressure.

In the hydraulic auxiliary power transmission, the hydraulic motor device(s) (500) drive(s) directly rotary elements of the mechanical transmission, namely a segment 821 of the main transmission shaft 820 in the drive system 1001 (FIG. 4), or transmission half-shafts 834 in the drive system 1002 (FIG. 5).

FIG. 4 shows a drive system 1001 including a hydraulic motor device 500 of the invention. This drive system 1001 includes a hydraulic auxiliary transmission 900 that is presented below.

As indicated above, the gearbox 810 has an outlet shaft 812 serving for the mechanical main transmission 800. In addition, a secondary outlet shaft 910 is also connected to said gearbox 810 and is thus driven in rotation. The secondary shaft 910 is connected to a hydraulic pump 920 that it, in turn, drives. The pump 920 is connected to the hydraulic motor device 500 via a hydraulic circuit comprising a feed circuit 922 and a discharge circuit 924 for the hydraulic motor device 500. By means of these circuits, the pump 920 feeds the hydraulic motor 500 with fluid under pressure.

Thus, a fraction of the drive power transmitted by the engine 400 to the gearbox 810 is diverted to the hydraulic pump 920 via the shaft 910. The pump 920 converts the rotary mechanical energy into fluid energy in the form of a fluid pressure. The fluid under pressure drives the hydraulic motor device 500 in rotation, the hydraulic motor device then in turn converting this drive power back into mechanical power by driving in rotation a shaft segment 821 on which it is placed.

This segment 821 is a central segment of a string of three segments 821 connected together in succession via flanges 840 and constituting the main shaft 820 of the mechanical main transmission.

Advantageously, the extra mechanical power delivered by the hydraulic motor device 500 can either replace or supplement the power transmitted directly by the motor 400 to the outlet shaft 812 of the gearbox 810. This extra power delivered by the hydraulic transmission can be used, in particular, when the shaft 820 is rotating at low rotation speeds at which the performance delivered by the engine 400 in association with the gearbox 810 is not sufficient.

Operation of the hydraulic transmission 900 is controlled by an electronic control unit (ECU) 460. Said unit is connected to the hydraulic motor device 500, to the pump 920, and to the ADC data bus of the vehicle via data interchange links 437, and makes it possible to monitor and control operation of the hydraulic transmission 900.

The vehicle drive system 1002 shown in FIG. 5 also includes a hydraulic auxiliary transmission analogous to the auxiliary transmission of FIG. 4. Since the general operating principle of a hydraulic auxiliary transmission, in which a fraction of the mechanical power is directed towards a pump for actuating an assistance hydraulic motor, is known (and is also indicated with reference to FIG. 4), in FIG. 5 only the hydraulic motor devices 500 of the hydraulic auxiliary transmission are shown.

In the drive system 1002 of FIG. 5, the main transmission shaft 820 has a single segment only.

In this drive system, the extra drive supplied by the hydraulic transmission is delivered to the half-shafts 834 rather than to the main transmission shaft 820. Each of the half-shafts has a hydraulic motor device as described above.

Thus, in this drive system, the transmission shaft segments incorporated into the hydraulic motor devices 500 are secondary transmission shafts 834, connected to a main transmission shaft 820 of the mechanical transmission 800.

Advantageously, the drive system 1002 makes it possible to exert anti-spin action to prevent wheel spin that can occur when an axle 830 including a differential is used on slippery ground. To this end, the drive system makes it possible to monitor or to control the speeds of rotation of the motors of the two hydraulic motor devices 500 by control means that are in particular hydraulic control means, thereby making it possible to guarantee that drive torque is exerted on at least one of the two wheels, even if the other one is spinning. 

1-16. (canceled)
 17. A hydraulic motor device, in particular for assisting the mechanical transmission of a vehicle, which hydraulic motor device includes a hydraulic motor that comprises a cylinder block having cylinders slidably receiving radial pistons, and a reaction cam for the pistons, the cylinder block and the cam being mounted to move in rotation relative to each other about an axis of rotation, the motor further comprising a fluid distributor for the cylinders, a portion of which distributor is constrained in rotation with the cam, and a distribution coupling mounted to be stationary relative to a reference frame of the vehicle and for connecting the distributor to a fluid feed and to a fluid discharge, the motor having a through passage disposed along the axis of rotation for the purpose of receiving a transmission shaft segment, wherein the motor can take up a declutched configuration in which the pistons are radially uncoupled from the cam, the cylinder block is stationary relative to the reference frame of the vehicle, while the cam is rotary, and the cam is constrained in rotation with a transmission shaft segment that is disposed in said through passage.
 18. A device according to claim 17, further including a drive part secured to the cam and that is suitable for co-operating with said transmission shaft segment to drive said shaft segment in rotation.
 19. A device according to claim 18, wherein the drive part and the transmission shaft segment are formed integrally as a single part.
 20. A device according to claim 17, wherein, on its periphery, the hydraulic motor has a connection portion adapted to transmit torque.
 21. A device according to claim 17, wherein the distribution coupling has a sleeve portion whose inside periphery forms a portion of the through passage.
 22. A device according to claim 21, wherein the distributor is disposed around said sleeve portion.
 23. A device according to claim 21, wherein the sleeve portion extends in an internal passage in the cylinder block.
 24. A device according to claim 21, wherein the sleeve portion co-operates with at least one rotary support bearing for supporting the shaft segment in rotation relative to the motor.
 25. A device according to claim 17, wherein the distributor is disposed on the other side of the cylinder block from the fluid feed and fluid discharge.
 26. A device according to claim 25, wherein the distributor is connected to the fluid feed and to the fluid discharge via ducts passing through the cylinder block between the cylinders.
 27. A device according to claim 17, wherein the distributor is disposed on the same side of the cylinder block as the fluid feed and fluid discharge.
 28. A device according to claim 17, wherein the cylinders extend radially relative to the axis of rotation, and the cam surface of the reaction cam is disposed around the cylinder block and faces towards the axis of rotation.
 29. A device according to claim 17, wherein the distributor for the cylinders is constrained in rotation with the cam.
 30. A vehicle drive system, comprising a mechanical transmission operated by at least one main engine or motor, and a device according to claim 17, the transmission shaft segment being a portion of the mechanical transmission.
 31. A vehicle drive system according to claim 30, in which system the transmission shaft segment is a portion of a main transmission shaft connected to the main engine or motor.
 32. A vehicle drive system according to claim 30, in which the transmission shaft segment is a portion of a secondary transmission shaft that is connected to a main transmission shaft of the mechanical transmission. 